Event Promise

event-promise.ts

import { EventEmitter } from "events";

export default function eventPromise<T>(emitter: EventEmitter, resolveEvent: string, rejectEvent: string): Promise<T> {
  return new Promise<T>((resolve, reject) => {
    let resolveHandler = (evt) => {
      resolve(evt);
      emitter.removeListener(resolveEvent, resolveHandler);
      emitter.removeListener(rejectEvent, rejectHandler);
    }
    let rejectHandler = (evt) => {
      reject(evt);
      emitter.removeListener(resolveEvent, resolveHandler);
      emitter.removeListener(rejectEvent, rejectHandler);
    }
    emitter.on(resolveEvent, resolveHandler);
    emitter.on(rejectEvent, rejectHandler);
  });
}

example.ts

import * as http from "http";
import * as WebSocket from "ws";
import eventPromise from "./event-promise";

export async function openWebSocket(url: string): Promise<WebSocket> {
  let ws = new WebSocket(url);
  return eventPromise(ws, "open", "error").then(() => ws);
}

export async function getResponse(options: any): Promise<http.IncomingMessage> {
  let request = http.get(options);
  return eventPromise<http.IncomingMessage>(request, "response", "error");
}

export async function readResponseBody(response: http.IncomingMessage): Promise<string> {
  let body = "";
  response.setEncoding("utf8");
  response.on("data", (chunk) => {
    body += chunk;
  });
  return eventPromise(response, "end", "error").then(() => body);
}

class ResponseError extends Error {
  status: number;
  constructor(message: string, status: number) {
    super(message);
    this.status = status;
  }
}

export async function get(options: any): Promise<string> {
  let response = await getResponse(options);
  let status = response.statusCode;
  let body = await readResponseBody(response);
  if (status !== 200) {
    throw new ResponseError(body, status);
  }
  return body;
}

export async function getJSON<T>(options: any): Promise<T> {
  let body = await get(options);
  return JSON.parse(body);
}

An utility function for creating a promise of an event with example code that it cleans up.